In general, direct dye products last only 6-10 shampoos and are hence known as semi-permanent. However, many consumers want more permanent results, and therefore default to oxidative dye products that contain hydrogen peroxide or other oxidants. Providing vibrant violet-blue to blue colors is challenging, because many of the vibrant dyes are not sufficiently stable for permanent haircolor compositions, and those that are stable to hair dyeing conditions are not sufficiently vibrant. The direct dye compounds and the method described herein can be used in either direct dye or oxidation dye products.
The permanent alteration of the color of keratinous fibers, in particular human hair, by the application of hair dyes is well known. In order to provide the consumer with the shade, longevity, and the intensity of color desired, an oxidative coloring process involving complex chemical reactions is utilized. Permanent hair dyeing formulations typically comprise primary intermediates (also known as oxidative hair dye precursors or developers) and couplers (also known as color modifiers or secondary intermediates). These dye precursors are sufficiently small, polar and soluble to diffuse into the hair shaft where, once activated by an oxidizing agent under basic conditions, such as hydrogen peroxide, the primary intermediates react with other dye precursors, e.g., couplers, to form larger colored chromophores in the hair shaft. The chromophores formed in the hair shaft do not readily diffuse out of the hair during subsequent washing with water and/or detergents because they are bigger, less polar and soluble than dye precursors that diffused in.
Hair colorant products are typically sold in the form of kits containing a dye component (e.g., a dye solution) and an oxidizing component (e.g., a hydrogen peroxide solution). In use, the dye component is mixed with the oxidizing component and the resultant mixture is applied to hair. When the two components are mixed, oxidizing agents present in the oxidizing component begins to oxidize primary intermediates present in the dye component and the oxidized primary intermediates begin to react with couplers to form chromophores. Since coloring hair is one of the beauty routines, it is highly desirable that the dyeing process, excluding bleaching, be rather a physical process, which would allow it to be aligned with many other beauty routines such as applying lip color and facial touchups. The challenge is to still meet all of the other requirements of hair color (e.g., washfastness, resistance to perspiration, little or no bleeding of color from the hair when it is wet, evenness, etc.).
Many attempts have been made by the hair color industry to enhance the washfastness of direct dyes by either forming a covalent bond between chromophore and proteins inside hair or increasing the number of binding sites, typically cationic centers, on the chromophore. However, each attempt has its drawbacks. The approach through covalent bonding does not differentiate proteins in hair from skin. The approach through multiple binding sites on the dyes (i.e. multiple positive charges to interact with negative sites on hair, either by bonding several monocationic dyes together or by installing multiple cationic centers on a single chromophore) runs into the obstacles of uneven color due to uneven damage (negative charges) along the length of the hair fibers and reduced dye penetration into hair fibers because the dyes are typically at least twice as large as common oxidative dye precursors. An increase in the number of binding sites minimizes bleeding and color loss caused by rinsing by providing stronger hair-chromophore interactions. However, the same strong binding force to the cuticle also prevents the chromophores from penetrating deep into the cortex of hair, because it is difficult for dyes with multiple positive charges to diffuse through negatively charged networks of keratin proteins. Additionally, since polycationic dyes remain bound to the hair surface rather than penetrating into the fiber, it is difficult to produce dark shades, due to limited binding sites on the surface of hair.
In dye chemistry, use of heterocyclic components in the chromophore leads to pure, intense colors. Pure, intense yellows are particularly important in hair dyeing, because they are necessary to deliver attractive gold and warm shades that are rich and brilliant rather than muted, as when less pure colors are used. They not only are needed for adding brightness, but they also can be used to give the appearance of lightening. For yellow direct dyes, one of the challenges is that some the heterocyclic moieties in dyes, such as thiazolium and pyridinium can be unstable under in-use conditions, particularly in the presence of oxidants like hydrogen peroxide.
Accordingly, there is a need for a direct dye compound with improved washfastness without the drawbacks previously described, and these imidazolium azo dyes meet all the criteria of color, stability, and fastness to be used as permanent hair dye compounds.